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a Dep. of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078-6027 USA
b Dep. of Hort. and Crop Science, The Ohio State University, Columbus, OH 43210 USA
bgregor{at}okstate.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Abbreviations: B, blue photon flux (400–500 nm) • G, green photon flux (500–600 nm) • FR, far-red photon flux (700–800 nm) • PAR, photosynthetically active radiation • PF, photon flux • PPF, photosynthetic photon flux (400–700 nm) • PPFFR, photosynthetic photon flux plus far-red photon flux (400–800 nm) • R, red photon flux (600–700 nm)
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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410, 430, and 660 nm (French, 1961). Chlorophyll b absorbs most effectively at 430, 455, and 640 nm, and carotenoids, including xanthophylls, absorb best in bands centered near 450 nm.
Phytochrome, a photomorphogenic pigment implicated in cell elongation, chloroplast development, and carotenoid biosynthesis (Harding and Shropshire, 1980), absorbs wavelengths preferentially at 660 and 730 nm (Grant, 1997). Using these findings, PAR can be further divided into high-activity and low-activity wavelengths based on pigment absorption bands. Photosynthetically active radiation from 400 to 500 nm, referred to as blue light, and PAR from 600 to 700 nm referred to as red light, is active for photosynthesis, photomorphogenesis, and chlorophyll synthesis (Blackwell, 1966). Photosynthetically active radiation from 500 to 600 nm, generally called green light, is basically inactive for plant growth and development. Far-red irradiance occurs in a spectral band from 700 to 800 nm and is not active for photosynthesis but strongly influences photomophogenesis (McMahon et al., 1991; Casal and Sanchez, 1994).
Shade, regardless of its source, reduces PAR and alters spectral quality, affecting plant photosynthesis and photomorphogenesis. Spectral quality, the relative intensity of spectra included in available irradiance, may have significant effects on turfgrass development. Low light intensities result in morphological and physiological conditions detrimental to turfgrass growth and development (Dudeck and Peacock, 1992). The severity of these conditions may be affected by low levels of irradiance, by spectral quality of irradiance, or both.
McBee (1969) reported that growth of bermudagrass (Cynodon dactylon L.) improved when blue light was present and red light was filtered compared with when red was present and blue was filtered. McKee (1963) used a color temperature meter, an instrument that measures blue and red spectral energy, to characterize spectral quality in the shade of buildings, the shade of tree canopies, and the shade of low-growing herbaceous plants. Blue light was enriched compared with red light in deciduous shade and in building shade, but declined in conifer shade and in dense herbaceous shade. McKee (1963) linked these responses to increasing shade density, suggesting that blue light was enriched relative to red light in open shade and restricted in dense shade. Using a similar instrument, Gaskin (1965) found no difference between proportions of red and blue light under green saran shade cloth compared with tree shade if light quantity was between 25 and 75% of full sun. These results are curious because chlorophyll absorption suggests that more blue light should be filtered by an herbaceous canopy than would be filtered by shade cloth. Gaskin (1965) also suggested that building shade filtered less blue light than oak (Quercus alba L.) and maple (Acer spp.). Vezina and Boulter (1966) demonstrated a reduction in red and blue wavelengths and a proportionate increase in green and far-red wavelengths under a deciduous forest canopy but found a "neutral" shade with respect to spectral quality under a coniferous forest canopy. The results of these studies appear contradictory in some cases, and further research is warranted to help explain the reasons for these apparent contradictions.
Recent research has focused on assessing spectral quality within tree canopies (Grant et al., 1996; Gilbert et al., 1995) or within forests (Turnball and Yates, 1993; Baldocchi et al., 1984). These studies provide valuable information concerning spectral intensities available for tree growth and for plant adaptation in deep shade, but they provide little information concerning spectral quality at ground level in relatively open shade. Global irradiance, the radiant energy available for plant growth, is a combination of direct, diffuse, and reflected solar radiance. Diffuse irradiance results from the scattering of solar spectra by atmospheric aerosols and other molecules (Brine and Iqbal, 1983). This diffusion occurs when molecular size and wavelength are nearly equal and primarily affects blue wavelengths (Gates, 1966). Diffusion causes the blue color of the sky and enables blue irradiance to strike the earth's surface at any angle originating from the sky hemisphere. Diffuse irradiance may contribute up to 25% of the blue irradiance present in full sun at a specific ground location (Ustin and Curtiss, 1990). Reflectance occurs when solar radiance is deflected by a nearby surface. The intensity and spectral quality of reflectance at any specific location is controlled by the proximity and origin of the reflecting material (Boer, 1977). Because of the influence of diffuse irradiance, which may strike the earth's surface at almost any angle originating from the sky hemisphere, infiltrating the shade created by a single tree or small group of trees, and because of reflection occurring within a tree canopy, spectral quality within tree canopies is not consistent with spectral quality beneath a canopy. Few plants are adapted for growth under forest canopies, and a forest environment is not consistent with spectral quality in shaded situations where horticultural plants are grown.
We now know that solar energy, often referred to as light in earlier publications, has both energy and particle distribution. Light particles, called photons or quanta, vary in energy content. A blue photon is at a higher energy level than a red one, yet both are equal in physiological potential. For that reason, energy measurements made in the 1960s may not fully explain spectral quality in shade. It is not the intention of this project to test the validity of previous studies but to discover a commonality among those studies that explains apparent discrepancies. Additional information is needed to determine spectral quality in the shade of a single tree or building where turfgrass and other horticultural plants may be cultivated. This information is important for understanding plant responses in shade and for implementation of plant management strategies in shade.
Ratios of blue quanta to far-red quanta and red quanta to far-red quanta influence plant physiology and morphology and have not been investigated in shade conducive to the growth of horticultural plants. Field observations suggest that turfgrass subjected to shade during the morning hours declines more readily than turf subjected to shade in the afternoon. Sophisticated equipment is available to measure radiant spectra, and techniques for conversion of radiant energy to quanta have been rigorously tested. The purpose of this study was to assess the spectral quality of deciduous shade, coniferous shade, building shade, and full sun in a natural environment common to turfgrass growth throughout a day and throughout a growing season in Columbus, OH.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Season-Long Evaluation
Spectra were acquired in the shade of a single tree or building and in full sun at the same location each time and in the same order each hour. Direct solar irradiance was measured after passing through approximately the same portion of tree canopy for each acquisition. Because of this protocol, the distance from the tree trunk where spectra were acquired varied with the position of the sun in the sky at the time of acquisition. During solar noon, spectra were acquired very close to the trunk (2 m) and direct irradiance was required to pass through a large portion of tree canopy before reaching the detector. In early morning or late afternoon, spectra were acquired much farther from the tree trunk (up to 20 m) and the canopy density filtering direct solar irradiance was reduced because of sun angle. Additional distance alterations were necessary due to seasonal variation in sun location. Adjusting the location of acquisition to coincide with the angle of direct irradiance and a specific portion of tree canopy provided a level of consistency necessary to measure the spectral influence of a single tree. This protocol provided a situation similar to a patch of turf or a low-growing plant shaded by a line or circle of trees throughout the day. The trees used to provide shade were a Norway spruce (Picea abies L.) 8 m tall, and a black walnut (Juglans nigra L.), 10 m tall, at the Research Center and a Norway spruce, 7 m tall, and a Sycamore (Platunus occidentalis L.), 12 m tall, at Campus. The coniferous trees were chosen for accessibility of location and uniformity between the Research Center and Campus. The deciduous trees were chosen for minimal canopy density and proximity to the conifer trees. Sunny or cloudy weather was considered a random occurrence and a part of the natural environment, and scans were made regardless of cloud conditions. Spectra were not acquired during periods of rain because moisture affected the accuracy of the sensing mechanism. When rain interfered with data collection, spectra were acquired on the next dry day. A total of 13 acquisitions per day for 12 d were used for evaluation at the Research Center and the same number of acquisitions were made each day for 7 d at Campus. Acquisitions at the Research Center were made a day later than acquisitions at Campus.
Morning and Afternoon Effects
Acquisitions made at 2, 3, and 4 h before solar noon and acquisitions made at 2, 3, and 4 after solar noon in full sun throughout the course of the study (April–September) were used to compare morning and afternoon irradiance.
Data Analyses
Radiant energy measurements (W m-2 nm-1) were collected for each wavelength from 300 to 850 nm at 5-nm increments. Photosynthetic photon flux (PPF; mol m-2 s-1) was calculated by adjusting radiant energy to quanta and integrating from 400 to 700 nm. Photon flux (PF; mol m-2 s-1) was calculated for blue irradiance (B) from 400 to 500 nm, for green irradiance (G) from 500 to 600 nm, and for red irradiance (R) from 600 to 700 nm in the same manner. These spectral ranges included both high activity (B + R) and low activity (G) quanta based on spectral absorption of plant pigments. A band from 700 to 800 nm was used to calculate far red photon flux (FR). Photosynthetic photon flux plus far-red photon flux (PPFFR) was calculated by adding PPF and FR. Photosynthetic photon flux plus far-red photon flux was used as a divisor to calculate the relative intensity of B, G, R, and FR in full sun and deciduous, coniferous, and building shade. Analyses of variance were performed to compare photosynthetic proportion [(B+R)/PPFFR], blue photoreceptor proportion (B/FR), and phytochrome proportion (R/FR) in deciduous shade, coniferous shade, and building shade. Time and month were used as blocking criteria to limit variation due to daily and seasonal changes in spectral density and quality. Mean spectral responses were separated using least significant difference. Significance was tested at the P < 0.05 level for all statistical analyses.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Morning and Afternoon Effects
Changes in PPF at the Research Center occurred parabolically throughout each day and throughout the season (Fig. 1)
. Solar radiation under clear sky normally increases slightly after solar noon (Salisbury and Ross, 1992). Conversely, this study demonstrated a significant decline in PPF after solar noon, probably because of increased cloud cover. Average PPF for spectra acquired in full sun at 2, 3, and 4 h before solar noon at the Research Center from April through September was 958.7 mol m-2 s-1 and for 2, 3, and 4 h after solar noon was 730.7 mol m-2 s-1. Continental climates, such as that in Columbus, OH, tend to have more cloud cover in the afternoon than in the morning during the growing season. Cloud cover scatters solar radiance and may affect spectral quality (Grant et al., 1996). Across the season and in full sun, B/PPFFR was significantly lower in the morning (0.1994) than in the afternoon (0.2044), R/PPFFR did not change significantly (A.M. = 0.3808; P.M. = 0.3685; P = 0.0643), and FR/PPFFR was greater in the morning (A.M. = 0.2769; P.M. = 0.2699). Although B/PPFFR increased in the afternoon, the proportion of (B + R)/PPFFR available to plants remained constant during a day (P = 0.0898). The R/FR was not affected by morning and afternoon periods, but B/FR increased significantly in the afternoon. This increase in B/FR is consistent with results predicted for cloud cover (Grant et al., 1996). Under cloud cover, B radiance was proportionately higher than under clear sky because of diffusion of B and cloud reflection of other wavelengths. The FR was proportionately lower as a result of absorption of FR by water vapor and cloud reflection of direct FR (Gates, 1960). Because cloud cover did not significantly affect R/FR and because B during daylight is normally in a saturating condition for B photoreceptors in full sun, these differences between light quality in the morning and afternoon may not have a major impact on plant development (Bell and Danneberger, 1999). However, persistent periods of cloud cover may affect photosynthetic production if PPF falls below light saturation points for desirable plant species (Fig. 1). Cloud conditions may cause plant responses to morning and afternoon shade to differ among climatic regions. Significant loss of PPF in the afternoon combined with plant responses to spectral quality in vegetative shade may be sufficient to reduce turf quality in morning shade compared with afternoon shade in some regions.
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750 and 780 nm. In fact, FR/PPFFR fluctuated wildly between 700 and 800 nm in all shade types and full sun.
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From calculated amounts of B, R, and FR in PPFFR, the ratios of B/FR and R/FR in full sun and shade were predictable (Table 1). The B/FR was greatest in building shade because diffuse and reflected diffuse B entered the shade area but direct FR did not. The B/FR was least in conifer shade because direct FR entered conifer shade freely. Conifer shade did not vary from full sun because the intensity of both B/PPFFR and FR/PPFFR increased. Plant responses controlled by blue photoreceptors are therefore unlikely to occur in dense, vegetative shade unless influenced only by B independent of FR and phytochrome state. The R/FR was greatest in full sun and least in conifer shade. This ratio varied by shade source because diffuse irradiance was not a primary factor. Direct FR penetrated tree canopies, but direct R was reflected or absorbed. Direct FR did not penetrate a building, consequently R/FR varied between building shade and vegetative shade. Because of these conditions, plant responses controlled by phytochrome may differ among shade types and full sun provided they are not influenced by B photoreceptors.
These findings support those of McKee (1963), Gaskin (1965), and Vezina and Boulter (1966). As McKee (1963) discussed, blue and red spectra were influenced by shade density. McKee (1963) found no difference for blue/red light in deciduous tree shade and building shade because shade density was similar. Gaskin (1965) found differences between these shade sources because shade density was not consistent. In a deciduous forest, Vezina and Boulter (1966) found a reduction in both blue and red spectra that can be explained by the inability of diffuse blue to penetrate a forest canopy from any direction. This project does not support additional findings of Vezina and Boulter (1966). In this study, Conifer and deciduous shade both differed in spectral quality compared with full sun.
Recent research has demonstrated that starch and sucrose synthesis and degradation (Doelger et al., 1997; Dewdney et al., 1993), NO3 uptake, reduction, and utilization (Teller et al., 1996; Kamiya, 1995; Lopez-Figueroa and Ruediger, 1991), stem and hypocotyl elongation (Ahmad and Cashmore, 1997; Casal and Sanchez, 1994; McMahon et al., 1991), leaf area expansion (Eskins, 1992; Van Volkenburgh et al., 1990), and cell division (Furuya et al., 1997; Zandomeni and Schopfer, 1993; Muenzner and Voigt, 1992) are affected by the proportions of B, R, and FR, but these relationships have not been fully elucidated. An antagonistic relationship may exist between blue photoreceptors and phytochrome. This relationship is not fully defined and little information exists concerning plant response to B, R, and FR relationships in nature. Because of the possible existence of a blue photoreceptor and an interaction between this receptor and phytochrome, it is beyond the scope of this study to predict plant responses on the basis of spectral quality in shade under natural conditions. However, it is believed that shaded turfgrass produces longer, thinner, low-density leaf blades that grow more vertically than sun plants. Root to shoot ratios decrease, cell walls and cuticles decrease in thickness, and chloroplasts are smaller in shaded environments (Dudeck and Peacock, 1992). Research under natural light conditions supports the existence of these responses in nature (Casal and Sanchez, 1994; Ballare et al., 1991; Kasperbauer, 1971). Buisson and Lee (1993), working with papaya (Carica papaya L.), demonstrated that at least some of these responses were influenced by light quality independent of light quantity. Research on turf in the field also demonstrates the existence of physiological or morphological plant responses due to changes in spectral quality (McVey et al., 1969).
The spectral changes between shade and full sun reported here appear small because they have not been manipulated by artificial environments. Turf will not grow in a natural environment where irradiance is reduced to a point necessary to induce, what might be considered, a major change in spectral quality. Many grasses would not survive the conifer shade density reported here unless exposed to full sunlight for a portion of each day. For instance, the light compensation point for perennial ryegrass (Lolium perenne L.) is reached for only an average of 2.5 h each day in conifer shade (Fig. 1). The light saturation point for this species was not reached (on average), even in sparse deciduous shade (Azcon-Bieto et al., 1981). The light saturation point (1000 mol m-2 s-1) for creeping bentgrass (Agrostis palustris Huds.) and annual bluegrass (Poa annua L.) was not reached in shade, and light saturation for Kentucky bluegrass (500–600 mol m-2 s-1; Poa pratensis L.) was reached for only a short time near solar noon in deciduous shade (Gaussoin, 1988). Yet, it is believed that turfgrasses do exhibit shade responses in nature due to changes in spectral quality. Additional field research is required to determine how and to what extent these responses are affected by relationships among B, R, and FR.
The results of this study demonstrated that reasonable evidence exists to indicate differential plant metabolism in shade compared with full sun and in building shade compared with tree shade. In open shade, B in PPFFR was influenced by both shade density and shade source. The B/PPFFR increased with increasing shade density and decreased in vegetative shade compared with building shade. Red photon flux in PPFFR decreased with increasing shade density. Far-red photon flux in PPFFR was influenced by shade source, increasing in vegetative shade and decreasing in building shade. Continued research may lead to models that accurately predict plant responses on the basis of the interaction of spectral light levels in specific shade environments. This study provides information that can be useful to turfgrass managers, but more importantly, to turfgrass researchers in pursuit of model systems for turfgrass management that balance the use of trees and turfgrass in a landscape.
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ACKNOWLEDGMENTS |
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Received for publication April 20, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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